A common means of monitoring electrical activity of body functions in humans is via receptors, such as electrodes, applied to the patient's skin. Monitoring brain activity, in particular, typically requires a plurality of electrically-connected receptors to be applied to predetermined, anatomically-precise sites on the patient's head.
Where the goal of monitoring electrical activity in the brain is to provide access to the particular recording sites that provide the best determination of anesthesia under a broad range of surgical anesthetics, a number of challenges are presented. First, the proper placement of receptors for electrical signals is essential to registering electrical activity in a fashion that can be interpreted medically. Second, the receptors must be able to maintain electrical contract with the skin throughout a possibly prolonged monitoring cycle without undue intervention by the clinician or anesthesiologist. In particular, during a surgical procedure the patient may have to be monitored for eight or occasionally ten hours. Maintaining proper impedance at the interface between the receptor and the skin is also very important in assuring readable and interpretable signals. Third, integrity of the sensitive conductors in the appliance must be ensured during hook-up and monitoring. If the conductors become shorted during set up or damaged while in place, the efficacy of the monitoring process may be compromised.
Heretofore, a large number of electrodes have been used to record electrical activity of the human brain. Typical monitoring activities have employed 21 electrodes mounted on the patient's head according to specific systems, primarily the International 10-20 system. It is known to employ electrodes in sets of 21, as for example in U.S. Pat. No. 5,497,934.
For certain applications, however, it is natural to conjecture that such a large number of electrodes might be unnecessary. Indeed, where the purpose of monitoring patient brain activity is to trace the level of consciousness in a patient receiving general anesthetic and thereby control the amount of anesthetic being administered, receptors that are not absolutely necessary for performing the desired patient monitoring function may become a distinct liability.
For example, the greater the number of receptors that are employed, the longer the task of preparing the patient. Where receptors are placed individually, the added set-up time is most pronounced. Placing individual electrodes one-by-one is a tedious process that involves measuring the location of the precise site for attaching each individual electrode, marking the location on the patient's head, preparing each attachment site, attaching the electrode, and wiring the electrode to the monitoring console. Electrode mounting sites are marked on the head of the patient with reference to the nasion, inion, and preauricular points. Marking the 21 standard mounting sites takes about 10 or 20 minutes using a tape measure and grease pencil. The sites are then cleaned with a cotton swab and treated with ground pumice stone or some other abrasive to provide good electrical contact to a conducting filler in a gold- or silver-plated cup electrode. Each electrode is attached individually to the patient's head using tape or collodion which solidifies as it dries and retains the electrode in the desired position on the head. This procedure for attaching 21 electrodes typically requires between 30 and 45 minutes. Then, since each electrode is attached and wired separately, a collection of 6 to 10 foot long wires radiate from the patient's head. These wires have to be individually connected to the console of the monitoring machine.
To a certain extent, the process of mounting 21 electrodes has been shortened by the introduction of devices to mark the scalp and to position electrodes on the patient's head. For example, U.S. Pat. No. 5,293,867 discloses headgear for marking the locations where electrodes are to be sited. Although this shortens the time required for the marking operation, it does not simplify any other aspects of the task--the patient's skin still has to be prepared at each of the 21 sites, and each of the 21 electrodes still must be mounted individually.
The greater the number of receptors, the greater the chance that one or more receptors will be improperly sited or improperly wired during set up. When a traditional approach is used, there is always the risk that the clinician will improperly locate one or more receptor sites.
Use of headgear does not entirely resolve these problems. For instance, U.S. Pat. No. 5,497,934 discloses an elaborate headgear design incorporating a large number of electrodes. While the device assists the clinician in reducing the time for placing and attaching the electrodes, headgear incorporating so many electrodes may have drawbacks associated with maintaining correct placement of all electrodes simultaneously. Since all of the electrodes are interconnected with elastic, moving one electrode may also move a plurality of adjacent electrodes, making it difficult to situate all of the required electrodes simultaneously.
Moreover, headgear such as that disclosed in U.S. Pat. No. 5,497,934 does not resolve the problems of maintaining good surface contact with the patient's skin. The task of preparing by hand the mounting sites for all 21 electrodes may be greatly complicated by the appliance's crisscrossing elastic strips.
A further problem with the headgear such as that disclosed in U.S. Pat. No. 5,497,934 is the possibility that wires can become shorted or improperly connected during set-up. Connectors for the electrodes are typically not integral to the headpiece, because the fixed length of a metallic wire or ribbon is not compatible with the requirement that the appliance contain elastic that stretches to fit the patient's head. The connectors are therefore separate from the headpiece, increasing the appliance's bulk and awkwardness. The appliance does not sit flat against the patient's head, increasing the potential that the headgear will interfere with activities occurring during the surgical procedure and increasing the possibility that the electrodes will become dislodged.
To resolve the problem of improper connection, headgear such as that disclosed in U.S. Pat. No. 5,497,934 usually envision that electrode leads will be bundled into a patient interface cable. The large number of electrodes make the resulting patient interface cable a 10 relatively stiff and cumbersome bundle of 21 6- to 10-foot long wires. A wiring cable of these proportions reduces flexibility in situating the console relative to the patient.
Moreover, the greater the number of receptors, the greater the likelihood that one or more receptors will become dislodged during monitoring. If the patient's head is moved or the patient interface cable is bumped, the conductors pull at and tend to dislodge the electrodes. The risk of a receptor becoming dislodged during an operation is particularly high when the patient is subject to intubation, as intubation can cause extensive movement of the patient's head. At times, the adhesive used to attach the electrodes is not strong enough to maintain both the proper electrical connection and the proper positioning throughout the monitoring session. Furthermore, even receptors that remain attached sometimes fail to make sufficiently good electrical contact with the skin to produce an effective signal. Movement in the cable may also shift the headgear and thereby dislodge the electrodes.
From the patient's perspective, headgear that uses an excessive number of receptors or employs suboptimal adhesion or conductive technology may aggravate the adverse perception of a medical situation. The set-up procedure can be long, tedious and complex which increases anxiety of the patient--particularly a patient that is being prepared for surgery. Clean up is also more complicated when a large number of receptors are used. The patient may have been marked with a grease pencil in multiple locations. Conductive gels used to enhance the electrical connection may be spread all over the patient's scalp.
Another obvious shortcoming associated with a large number of receptors is the expense. When the headgear utilizes 21 electrodes and contains the associated connectors, it becomes a fairly complex and costly device. Accordingly, it is common to retain headgear for use on multiple patients while replacing the electrodes with each use. The drawbacks of reuse are particularly acute in the environment of an operating room where cleanliness is an issue. The process of cleaning or sterilizing the headgear could compromise some of the initial properties of the materials, for example, elastic will tend to lose its elasticity when heated or subjected to particular chemicals. If the elasticity of the headgear is affected by age or cleaning (particularly where the loss of elasticity is not uniform in all sections of the headgear), then the headgear will lose its ability to position the electrodes correctly and to exert adequate force to maintain the desired electrical contact during monitoring. It would be ideal if the headgear was so inexpensive as to be disposable.
It has been suggested that headgear designed to accept a large number of electrodes could be utilized with a lesser number of electrodes and would provide a level of service equivalent or similar to an appliance designed specifically for a reduced number of electrodes. Regardless of the number of electrodes mounted in a headpiece designed for a large number of electrodes, such as that disclosed in U.S. Pat. No. 5,497,934, the task of siting electrodes properly on the patient's head could be adversely affected by the many crisscrossing elastic strips that transmit movement and force from one section of the headpiece to another. The difficulties with accommodating hair on a patient would be exacerbated by such a close network of elastic strips. Furthermore, headgear designed to accept a large number of electrodes would in all likelihood remain wired for its full complement of electrodes. For these and other reasons, utilizing headgear designed for a large number of electrodes to monitor activity using a lesser number of electrodes is undesirable.
Accordingly, an object of this invention is to provide an appliance that uses a minimal number of receptors to monitor the brain activity of a human patient for the purpose of ascertaining the level of consciousness of the patient. Another object of this invention is to provide a convenient means of connecting the monitoring appliance to a console that will interpret the information being monitored with minimal interference with medical procedures and minimal safety risks. A further object of this invention is to provide a means whereby the clinician or anesthesiologist is able to quickly locate and prepare the exact sites where each of the receptors must be attached to the patient's head in order to be medically effective during monitoring without the necessity to first place then remove the appliance from the patient's head before preparing the skin surface or the necessity to work around tightly crisscrossed elastic strips. Another object of this invention is to provide a means of securing the receptors that adjusts easily to the human head in a wide variety of shapes and sizes. Another object of this invention is to avoid the use of glue or paste to secure the appliance on the patient's head. An additional object of this invention is to provide an appliance that is secure enough to stay reliably attached during typical surgeries, and procedures such as intubation. Accordingly, an object of this invention is to provide a means of securing the receptors to the patient's head that will withstand significant movement of the head at any time during the monitoring session without dislodging any receptor or interrupting gathering of the electrical signals by the receptors. Another object of this invention is to provide an appliance where the electrical circuitry for the receptors is integral to headpiece. A further object of this invention is to provide an appliance which is inexpensive to manufacture and which can be used as a medical disposable. An additional object of this invention is to provide an appliance that requires no preparation of the skin surface at mounting sites including those sites where scalp hair is present.